Electroforming has some advantages. For example, (1) ultrahigh-precision processing can be performed, (2) a metal product integrated with a base material can be manufactured, and (3) a precise duplicate of a prototype can be manufactured. Electroforming is utilized for manufacturing various types of metal products such as a copper foil for a printed circuit board, an outer edge of an electric shaver, a precision screen, a face of a wrist watch, and a mold for forming a compact disk.
Particularly in recent years, the needs for a fine metal product of an overall size on the order of microns have tended to increase, as represented by the miniaturization of parts caused by the miniaturization of electronic apparatuses, and it has been examined that electroforming is applied to the manufacture thereof.
Examples of the fine metal product manufactured by electroforming include one that uses a metal thin film formed on a base material by electroforming, together with the base material, in an integrated state and one that uses a formed metal thin film as an independent product by peeling off the metal thin film from a base material.
Although the former metal product occupies almost all the fine metal products at present, it is expected that the use and the demand of the latter metal product increase from now on.
The latter metal product is manufactured by preparing a mold for fine electroforming comprising an electrode portion having a fine shape corresponding to its plane shape, making the electrode portion of the mold serve as a cathode, making a metal thin film selectively grow on its surface by electroforming, and then peeling off the metal thin film, which has grown, from the electrode portion and recovering the peeled metal thin film.
As an example of the mold for fine electroforming used for such a method are one obtained by forming on a surface of a conductive substrate such as a metal plate a resist film having insulating properties, and having a lot of openings having a shape corresponding to the plane shape of a metal product to be manufactured and through to the surface of the conductive substrate by lithography or the like, and using as an electrode portion the surface of the conductive substrate exposed through the openings of the resist film.
In the above-mentioned mold, the resist film mainly composed of an organic material such as resin is weak and is liable to be damaged. Moreover, the thickness thereof is significantly larger than the thickness of the metal thin film formed by electroforming. Therefore, it is difficult to peel off the formed metal thin film from the surface of the conductive substrate without damaging the resist film.
In the above-mentioned mold, therefore, it is considered that the resist film, together with the metal thin film, is peeled off every time electroforming is performed, considering that a rate of recovery in peeling off the metal thin film from the electrode portion and recovering the peeled metal thin film is improved.
When such a recovering method is carried out, however, the mold is made unusable every time electroforming is performed, and must be newly re-formed. Therefore, the productivity of the mold and the metal product manufactured using the mold are low, so that the production cost is significantly high.
Therefore, the inventors have proposed a mold for fine electroforming 9 having a structure-shown in FIG. 4 (Japanese Laid-Opened Patent Publication JP2002-97591A).
The mold for fine electroforming 9 is one obtained by forming a lot of very small projections 91 each having a front end surface 91a corresponding to the plane shape of a metal product by lithography or the like on a surface of a conductive substrate 90 composed of a metal plate, then causing liquid resin to flow thereonto to cure the resin to form an insulation layer 92 which is sufficiently thicker and stronger than a resist film, and then polishing a surface of the insulation layer 92 to expose the front end surface 91a of the projection 91 and make the exposed front end surface 91a serve as an electrode portion.
In the mold for fine electroforming 9, the insulation layer 92 is sufficiently thicker and stronger than the resist film, as described above, and the front end surface 91a of the projection 91 and the surface of the insulation layer 92 are nearly flush with each other. The metal thin film is formed in a shape projected upward from the flush surfaces. Accordingly, the metal thin film can be recovered by peeling off the insulation layer 92 without practically causing damage thereto. Consequently, the one mold for fine electroforming 9 can be reused for electroforming many times.
When the above-mentioned mold 9 is used, however, the metal thin film may not, in some cases, be easily peeled off. The cause thereof is that a so-called anchor effect is produced between the surface of the mold 9 and the metal thin film.
That is, in the above-mentioned mold 9, the front end surface 91a of the projection 91 tends to enter a state projected very slightly from the surface of the insulation layer 92 depending on the difference in the ease of wear at the time of polishing between the metal and the resin or contraction at the time of curing of the resin, in a case where the resin is curable resin.
Alternatively, a very small clearance may, in some cases, occur between a side surface of the projection 91 and the insulation layer 92 depending on the difference in a coefficient of expansion therebetween, contraction at the timing of curing of the above-mentioned curable resin, or the like.
During electroforming, the metal thin film grows not only on the front end surface 91a, but also on the side surface of the projection 91 exposed by the projection or the clearance, and the grown metal thin film of the side surface produces an anchor effect, so that the metal thin film, which has grown on the side of the front end surface 91a, made to serve as a metal product is not easy to peel off.
The metal thin film has a microstructure. When there occur situations where the metal thin film is difficult to peel off, as described above, therefore, the metal thin film is easily deformed and damaged by a stress created at the time of peeling, and the manufacturing yield of the fine metal product composed of the metal thin film is significantly lowered.
Furthermore, when an attempt to force the metal thin film to be peeled off by a strong force is made, an excessive force is also applied to the mold 9. Therefore, the degradation of the mold 9 becomes fast.
Particularly, the insulation layer 92 more easily wears away by a stress created in peeling off the metal thin film, for example, as compared with the projection 91 made of metal even if it is formed of curable resin such as epoxy resin. When the wear progresses, the side surface of the projection 91 is further greatly exposed. Therefore, it may be not only further difficult to peel off the metal thin film because the above-mentioned anchor effect is increased but also impossible to obtain a metal product having a correct shape because the metal thin film grown not only on the front end surface 91a, but also on the side surface of the projection 91 becomes too large.
Furthermore, the insulation layer 92 is peeled off from the conductive substrate 90 over a wide area by the above-mentioned stress or the like so that the mold may be entirely unusable.
In order to improve the productivity of the metal product, it is preferable that the number of metal products which can be manufactured by performing electroforming once using one mold is made as large as possible.
Therefore, it is needed to make the number of projections 91 as large as possible in the above-mentioned mold 9. In order to sufficiently ensure the thickness of the insulation layer 92, however, the aspect ratio of the projection 91, that is, the ratio of the diameter to the height of the projection 91 must be significantly higher than one. Therefore, it is by no means easy to form a lot of projections 91 having such a high aspect ratio on the surface of the conductive substrate 90 with a high density even by a current high-precision processing technique such as lithography.
In the above-mentioned mold 9, therefore, the improvement in the productivity of the metal product has a limitation.